Distance Measuring Equipment (DME) is a transponder-based radio navigation technology that measures distance by timing the propagation delay of radio signals. The DME operates by transmitting to and receiving paired pulses from the ground station. The transmitter in the aircraft sends out very narrow pulses. These signals are received at the ground station and trigger a second transmission on a different frequency. These reply pulses are sensed by timing circuits in the aircraft's receiver that measure the elapsed time between transmission and reception. Electronic circuits within the radio convert this measurement to electrical signals that operate the distance and ground speed indicators. Tactical Air Navigation, or TACAN, is a navigation system used by military aircraft.
A typical DME system is composed of a UHF transmitter/receiver (interrogator) in the aircraft and a UHF receiver/transmitter (transponder) on the ground. DME transponders transmit on a radio channel frequency and receive on a corresponding channel frequency. The band is divided into 126 channels for interrogation and 126 channels for transponder replies. The interrogation and reply frequencies always differ by 63 MHz. The spacing of all channels is 1 MHz with a signal spectrum width of 100 kHz. The minimum operational performance standards for various DME systems include RTCA DO-189 for Airborne Distance Measuring Equipment (DME) Operating within the Radio Frequency Range of 960-1215 Megahertz, RTCA DO-181 C for Air Traffic Control Radar Beacon System/Mode Select (ATCRBS/MODE S) Airborne Equipment, and RTCA DO-282A for Universal Access Transceiver (UAT) Automatic Dependent Surveillance Broadcast (ADS-B)
To save part cost and size in a radio communication system in DME, it is desirable to use the lowest cost and slowest analog-to-digital converters that will properly sample the information band. Lower sample rates, however, cause interfering signals that are aliased onto the desired signal in the data conversion process if they are not properly reduced in amplitude prior to data conversion. Lower sample rates thus necessitate increases in radio part costs in the form of narrow band anti alias radio frequency (RF) filters needed to remove interfering signals. It is therefore desirable to separate the desired signal from interfering signals without incurring additional part cost in a radio communication system using low sample rate analog-to-digital converters.